In recent years, VLSI technology has been drastically progressed, and an SOI structure by which speeding up and low power consumption are realized has been attracted attention. In this technology, an active region (a channel formation region) of a field effect transistor (FET), which has been conventionally formed of a bulk single crystal silicon, is formed of a single crystal silicon thin film. It is known that a MOS (Metal Oxide Semiconductor) field effect transistor (MOSFET) fabricated using an SOI structure can reduce its parasitic capacitance more than a conventional one formed using a bulk single crystal silicon substrate, and such a MOSFET is advantageous for speeding up.
As a conventional SOI substrate, a SIMOX substrate, a bonded substrate and the like are known. For example, for an SOI structure of SIMOX substrate, oxygen ions are implanted into a single crystal silicon substrate, heat treatment at 1300° C. or higher is conducted to form a buried oxide (BOX) layer, so that a single crystal silicon film is formed on the surface. In the SIMOX substrate, oxygen ion implantation can be controlled precisely and thus a single crystal silicon thin film having an even thickness can be formed with high control; however, there is a problem in that long time period is needed for oxygen ion implantation to cause problems of time and costs. Further, there is another problem in that the single crystal silicon thin film is easy to be damaged in the oxygen ion implantation.
As an SOI structure of a bonded substrate, two single crystal silicon substrates (a base substrate and a bond substrate) are bonded to each other with an oxide film therebetween, and one of the two single crystal silicon substrates (the bond substrate) is thinned on its rear side (which is not a surface to be used for bonding), so that a single crystal silicon thin film is formed. As a method for thinning, there is proposed Smart-Cut (registered trademark) employing hydrogen ion implantation (e.g., Reference 1: Japanese Published Patent Application No. H5-211128), because it is difficult to form a uniform and thin single crystal silicon thin film by grinding and polishing.